Background: Childhood overweight and obesity represent growing public health challenges globally, driven by complex interactions between demographic, behavioral, and familial factors. This study assessed the prevalence and determinants of overweight and obesity among school-aged children and adolescents attending urban schools in the Aseer Region of Saudi Arabia (Abha and Khamis Mushait). Methods: A cross-sectional study was conducted among 413 urban school students aged 6–18 years. Anthropometric measurements were obtained using standardized procedures, and lifestyle behaviors were assessed through a structured questionnaire. Data were analyzed using descriptive statistics, Chi-square tests, and multivariate binary logistic regression. Variables with p < 0.05 in univariate analysis, alongside conceptually relevant predictors, were included in the final model. Adjusted odds ratios (AORs) with 95% confidence intervals (CIs) were reported. Results: The combined prevalence of overweight and obesity was high, particularly among older age groups and secondary-school students. Significant associations were found between BMI category and age, school level, parental education, and family history of obesity. Lifestyle behaviors including fast-food consumption, low fruit and vegetable intake, prolonged screen time, and short sleep duration showed strong relationships with overweight/obesity. In the adjusted model, key predictors included frequent fast-food intake (AOR = 2.74), low fruit/vegetable intake (AOR = 2.20), physical inactivity (AOR = 1.70), high screen time (AOR = 2.40), short sleep duration (AOR = 1.55), and positive family history of obesity (AOR = 3.10). Conclusions: Childhood overweight and obesity in the Aseer Region are influenced by both modifiable lifestyle behaviors and familial predisposition. Targeted interventions promoting healthy dietary habits, adequate physical activity, reduced screen time, and sufficient sleep are essential. Preventive strategies should prioritize adolescents and children with a family history of obesity to effectively reduce obesity risk.